This invention relates to an optical property measuring device and more particularly to an optical property measuring device for measuring the reflectance and transmittance of a sample to be measured such as a small-sized or minute beam splitter prism.
The beam splitter prism is an important optical element for use in, e.g. the sound recording, image recording and playback of a laser disk. This beam splitter prism, as shown in FIGS. 3 and 4, is shaped in a cube formed by bonding two 45.degree. prisms to each other or its modification and has a bonding face subjected to particular treatment so that incident light is separated into transmission light and reflecting light with a certain ratio. Incidentally, in FIGS. 3 and 4, 1 denotes the bonding face, and I, T and R denote incident light, transmission light and reflection light, respectively.
One method of measuring the reflectance of a certain plane in order to manage the optical property mentioned above is to use an optical system for absolute reflectance measurement, as disclosed in, e.g., HATSUMEI KYOKAI KOKAI GIHO No. 79-1150. This method, when a sample to be measured takes the form of a cube such as shown in FIGS. 3 and 4 and is minute in its size, requires a complicated mechanism and sophisticated managing technique in order to hold the optical system so that the incident light flux is correctly incident to the central portion of the sample and also the reflection plane (bonding face in the present case) is directed in a normal direction.
For example, when using the above measuring method, in order to measure a beam splitter prism having a very small size of about 3 mm.sup.3, the projected light flux should have a very small diameter of about 1.5 mm.phi.. Further, if a sample holder, i.e., means for holding the beam splitter prism should be arranged in a normal direction with an inclination of up to 1/200.degree. by arranging the beam splitter prism at a normal position with an error of about 0.2 mm. Otherwise, the reflection light comes out of the light receiving face of the integrating-sphere, thus providing some measuring error. On the other hand, the bonding face of the beam splitter prism, as seen from the perspective view of FIG. 5, may have a step displacement of 20.mu. or more because of the fabrication technique and of being not required as a product. If such a beam splitter prism is set in the planar sample holder, an inclination exceeding 1/200.degree. necessarily occurs. Namely, even if the sample is slightly inclindely set in the holder, poor reproducibility results. This makes it impossible to measure the sample at a high speed and high accuracy, thus disadvantageously providing different measurement values for the same products or samples although the same holding method is used.